Cellular communication systems provide not only voice services, but also mobile broadband services all over the world. As more and more applications executable on cell phones are emerging, which consume higher and higher amounts of data, demands for mobile broadband data services have been increasing exponentially, requiring operators of cellular communication systems to improve data throughput wherever and whenever possible.
As the spectrum efficiency for the point-to-point link approaches its theoretical limit, one way to improve data throughput of a cellular communication system is to split big cells into smaller and smaller cells. When cells becomes closer to each other, however, adjacent cell interferences become more severe, and the cell splitting gain saturates. Furthermore, it is becoming increasingly difficult and costly for the operators to acquire new sites to install base stations. Therefore, cell-splitting cannot fulfil the demands for mobile broadband data services.
Recently a new type of cellular communication system deployment, called Heterogeneous Network or HetNet in short, has been proposed. In a HetNet, an additional tier including multiple low-power nodes (LPNs) is added into the cellular communication system within the coverage area of an existing macro base station. The macro base station monitors, controls, and schedules communications with the LPNs in a master-slaves relationship in the HetNet in order to have better interference management and resource allocation, etc.
In a HetNet, not all of the possibly deployed nodes are needed all of the time. At low traffic loads, one or more deployed low power nodes may be switched off. Switching off unneeded low power nodes provides a reduction in network power consumption and a reduction of mutual interference between the nodes. In legacy versions of the 3GPP standard, a mechanism has been introduced to switch cells on and off. LTE base stations (eNodeB) can also send activation and deactivation requests to each other.
When a legacy small cell is switched off, the small cell is not transmitting the legacy signals, such as primary and second synchronization signals (PSS/SSS) and cell specific reference signals (CRS). When the small cell is switched off, legacy user equipment (UE) cannot detect the small cell or perform radio resource management (RRM) measurements on the small cell.
In a legacy system, a cell probing method may be implemented to allow UEs to identify small cells that are switched off. In a cell probing method, small cells transmit a signal, such as a PSS, SSS, CSR, etc. signal for a certain time period. A UE can detect and measure the transmitted signal for RRM measurements and/or to identify a small cell for handoff.